Light-emitting device

ABSTRACT

A light-emitting device includes a semiconductor layered structure; an upper electrode disposed on a portion of an upper surface of the semiconductor layered structure; a lower electrode disposed on a lower surface of the semiconductor layered structure in a region spaced from a region directly under the upper electrode, the lower electrode being reflective; and a protective film disposed continuously on a surface of the upper electrode and the upper surface of the semiconductor layered structure. A thickness of a first portion of the protective film, which is disposed at least in a region directly above the lower electrode, is smaller than a thickness of a second portion of the protective film, which is disposed continuously on the surface of the upper electrode and the upper surface of the semiconductor layered structure adjacent to the portion on which the upper electrode is disposed.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 15/462,362, filed on Mar. 17, 2017, which claims priority toJapanese Patent Application No. 2016-056443, filed on Mar. 22, 2016, thedisclosures of which are hereby incorporated by reference in theirentireties.

BACKGROUND

The present disclosure relates to a light-emitting device.

Japanese Laid-Open Patent Publication No. 2014-236070 discloses alight-emitting device in which a bonding electrode is disposed on anupper surface side of a semiconductor layered structure, a metalelectrode is disposed on a lower surface side of the semiconductorlayered structure, and a protective film is disposed on a surface of thesemiconductor layered structure.

SUMMARY

In the above-described light-emitting device, a protective film with auniform thickness is disposed on an upper surface of the semiconductorlayered structure. With this arrangement, in the case where theprotective film has a small thickness, the semiconductor layeredstructure may be deteriorated. If the thickness of the protective filmis increased, deterioration of the semiconductor layered structure canbe suppressed but, on the other hand, light from the semiconductorlayered structure is easily absorbed by the protective film, so that thelight extraction efficiency may be decreased. In view of this, one ofthe objects of the present disclosure is to provide a light-emittingdevice in which deterioration of the semiconductor layered structure isreduced and which has a high light extraction efficiency.

A light-emitting device according to one embodiment of the presentdisclosure includes: a semiconductor layered structure; an upperelectrode disposed on a portion of an upper surface of the semiconductorlayered structure; a lower electrode having light reflectivity anddisposed on a lower surface of the semiconductor layered structure in aregion a region of spaced from a region directly under the upperelectrode; and a protective film disposed to be continuous on a surfaceof the upper electrode and on the upper surface of the semiconductorlayered structure. A portion of the protective film in a region directlyabove the lower electrode has a thickness smaller than a thickness of aportion of the protective film that is continuous on the surface of theupper electrode and on the upper surface of the semiconductor layeredstructure in a region in the vicinity of a region in which the upperelectrode is disposed.

The above-described configuration can realize a light-emitting device inwhich deterioration of the semiconductor layered structure is reducedand which has a high light extraction efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view schematically showing the configuration of alight-emitting device according to a first embodiment.

FIG. 2 is a partial cross-sectional view taken along line A-A of FIG. 1,schematically showing the configuration of the light-emitting deviceaccording to the first embodiment.

FIG. 3 is a top view schematically showing the configuration of thelight-emitting device according to the first embodiment.

FIG. 4 is a partial cross-sectional view taken along line A-A of FIG. 1,schematically showing the configuration of a light-emitting deviceaccording to a second embodiment.

FIG. 5 is a partial cross-sectional view taken along line B-B of FIG. 1,schematically showing the configuration of the light-emitting deviceaccording to the first embodiment.

FIG. 6 is an enlarged top view of a region indicated by R1 in FIG. 3,schematically showing the configuration of the light-emitting deviceaccording to the first embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1 is a top view of a light-emitting device 100 according to thepresent embodiment. FIG. 2 is a schematic partial cross-sectional viewfor illustrating the configuration of the light-emitting device 100.FIG. 3 is a schematic top view for illustrating a region of thelight-emitting device 100 in which members disposed on the upper andlower surfaces of a semiconductor layered structure 10 are disposed.FIG. 4 is a partial cross-sectional view for illustrating theconfiguration of a light-emitting device 200 according to a secondembodiment. FIG. 5 is a partial cross-sectional view for illustratingthe configuration of a light-emitting device 100 according to the firstembodiment. FIG. 6 is an enlarged top view of a region indicated by R1in FIG. 3, schematically showing the configuration of the light-emittingdevice 100 according to the first embodiment. In FIG. 3 the hatchedregion indicates a region in which a lower electrode 12 is disposed in atop view, and does not indicate a cross section.

The light-emitting device 100 includes a semiconductor layered structure10, an upper electrode 11 disposed on a portion of the upper surface ofthe semiconductor layered structure 10, a lower electrode 12 havinglight reflectivity and disposed on the lower surface of thesemiconductor layered structure 10 in a region spaced from a regiondirectly under the upper electrode 11, and a protective film 13 disposedto be continuous on a surface of the upper electrode 11 and on the uppersurface of the semiconductor layered structure 10. A portion of theprotective film 13 in a region 12R directly above the lower electrode 12has a thickness smaller than a thickness of a portion of the protectivefilm 13 disposed continuously on the surface of the upper electrode 11and on a portion of the upper surface of the semiconductor layeredstructure 10 in the vicinity of a region in which the upper electrode 11is disposed.

With this arrangement, deterioration of the semiconductor layeredstructure can be reduced and light extraction efficiency can beimproved. This is illustrated in the description below.

In the light-emitting device 100, the upper surface of the semiconductorlayered structure 10 serves as a surface from which light is primarilyextracted. Surfaces of the semiconductor layered structure 10 arecovered with a transparent protective film 13 for reducing deteriorationof the semiconductor layered structure 10. However, a portion of lightfrom the semiconductor layered structure 10 is absorbed by theprotective film 13, so that the output can be decreased. In particular,in the light-emitting device 100, the upper electrode 11 and the lowerelectrode 12 are disposed in different regions in a top view. Therefore,when viewed from the top surface, the region directly above the lowerelectrode 12 tends to emit intense light. Thus, in the region directlyabove the lower electrode 12, a portion of the light is easily absorbedby the protective film. Meanwhile, moisture may enter through an openingdefined in the protective film 13, through which the power is to besupplied from an external device, and a portion of the protective film13 having a small thickness. Further, at the upper surface of thesemiconductor layered structure 10, current density in regions in thevicinity of the regions in which the upper electrodes 11 are disposed(hereinafter, may be referred to as “vicinity regions 13R”) is higherthan the other region, and accordingly optical density in this region isincreased. Due to moisture and high optical density as described above,portions of the semiconductor layered structure 10 in the vicinityregions 13R may be easily oxidized and deteriorated.

In view of the above, in the present embodiment, the thickness of aportion of the protective film 13 on the upper surface of thesemiconductor layered structure 10 in the region 12R directly above thelower electrode 12 is reduced, which allows for reducing absorption oflight by the protective film 13, so that the light extraction efficiencyis improved. Meanwhile, the thickness of a portion of the protectivefilm 13 on the surface of the upper electrode 11 and in the vicinityregions 13R is increased. With this configuration, in the vicinityregions 13R, where the semiconductor layered structure 10 tends to beeasily deteriorated, deterioration of the semiconductor layeredstructure 10 can be reduced. That is, in the present embodiment, thelight extraction efficiency can be improved while the reliability of thelight-emitting device 100 is maintained.

In the description below, the configuration of the light-emitting device100 is described with reference to the drawings.

The semiconductor layered structure 10 includes a p-side semiconductorlayer 10 p, an active layer 10 a and a n-side semiconductor layer 10 nin this order from the substrate 16 side, which is the lower side of thelight-emitting device 100, as shown in FIG. 2. In a top view, thesemiconductor layered structure 10 has a generally square shape witheach side of about 2 mm. For each of the n-side semiconductor layer 10n, the active layer 10 a and the p-side semiconductor layer 10 p, anitride semiconductor such as, for example, In_(X)Al_(Y)Ga_(1-X-Y)N(0≤X, 0≤Y, X+Y<1) can be used.

The upper surface of the n-side semiconductor layer 10 n of thesemiconductor layered structure 10 can be a roughened surface. With theupper surface that is roughened, the light extraction efficiency can beimproved. For example, when upper surface of the n-side semiconductorlayer 10 n is roughened by forming a plurality of protrusions, each ofthe protrusions preferably has a height of 0.2 to 3.0 μm, morepreferably has a height of 0.4 to 1.5 μm, for appropriately improvingthe light extraction efficiency.

The upper electrodes 11 are disposed on portions of the upper surface ofthe semiconductor layered structure 10 and are electrically connectedwith the n-side semiconductor layer 10 n as shown in FIG. 2. That is, inthe present embodiment, the upper electrodes 11 function as then-electrode.

The upper electrodes 11 each includes an external connecting portion 11a and an extended portion 11 b extending from the external connectingportion 11 a. The external connecting portion 11 a is a region to beconnected with an external member, such as a wire. The extended portion11 b is an electrode for diffusing an electric current supplied to theexternal connecting portion 11 a over a larger area. As shown in FIG. 1,a plurality of external connecting portions 11 a are disposed on theupper surface of the semiconductor layered structure 10, the extendedportion 11 b is disposed at each of the external connecting portions 11a. For the upper electrodes 11, a metal such as Ni, Au, W, Pt, Al, Rh,Ti, or the like, can be used.

The lower electrode 12 is disposed on a portion of the lower surface ofthe semiconductor layered structure 10 and is electrically connectedwith the p-side semiconductor layer 10 p. That is, in the presentembodiment, the lower electrode 12 functions as the p-electrode. In FIG.3, the lower electrode 12 is disposed in a region hatched with obliquelines sloping upward to the left. As can be understood from FIG. 3, thelower electrode 12 is disposed on the lower surface of the semiconductorlayered structure 10 in a region which is spaced from a region directlyunder the upper electrodes 11. With this arrangement, current can beeasily flown into portions of the semiconductor layered structure 10 inthe region other than the regions directly under the upper electrodes11, so that the current is diffused over a larger area in thesemiconductor layered structure 10. Accordingly, the emission region ofthe light-emitting device 100 can be increased.

The lower electrode 12 is configured to reflect light from thesemiconductor layered structure 10 toward the upper surface side, whichallows for improving the light extraction efficiency of thelight-emitting device 100. For this reason, the lower electrode 12 ispreferably made of a metal which has a high reflectance. For example, ametal such as Ag, Al, or an alloy whose main component is one or more ofthese metals can be used.

The protective film 13 has the function of protecting the light-emittingdevice 100. The protective film 13 is disposed on a portion of the uppersurface of the semiconductor layered structure 10 on which the upperelectrodes 11 are not disposed, and on a surface of each of the upperelectrodes 11. To secure connection with external members, theprotective film 13 is not disposed on a portion of each externalconnecting portion 11 a. At this region, an electrically-conductivewire, or the like, is bonded for electrical connection with an externalpower supply.

A portion of the protective film 13 in the region 12R directly above thelower electrode 12 has a thickness smaller than a thickness of a portionof the protective film 13 continuous on the surface of each of the upperelectrodes 11 and on the upper surface of the semiconductor layeredstructure 10 in the vicinity regions 13R. In other words, the portion ofthe protective film 13 disposed continuously on the surface of each ofthe upper electrodes 11 and the upper surface of the semiconductorlayered structure 10 in the vicinity regions 13R has a thickness (forexample, Ta and Tb shown in FIG. 5) greater than the thickness (forexample, Tc shown in FIG. 5) of a portion of the protective film 13 inthe region 12R directly above the lower electrode 12. With thisarrangement, deterioration of the semiconductor layered structure 10 canbe reduced, and light from the semiconductor layered structure 10 can beefficiently extracted. Note that the “thickness” of the protective film13 used in the present specification refers to a dimension of theprotective film in a direction perpendicular to a surface of each memberon which the protective film 13 is to be formed.

In the light-emitting device 100, a thickness of the protective film 13is reduced only in the region directly above the lower electrode 12.That is, in the other regions, the protective film 13 has a relativelylarge thickness. With this arrangement, the light extraction efficiencycan be further improved.

The protective film 13 has a relatively large thickness at the peripheryof the semiconductor layered structure 10 as shown in FIG. 2. A portionof the protective film 13 at the periphery of the semiconductor layeredstructure 10, particularly a portion of the protective film 13 over theupper and lateral surfaces of the semiconductor layered structure 10, islikely to have a smaller thickness. In view of this, forming protectivefilm 13 to have a relatively large thickness at the periphery of thesemiconductor layered structure 10 allows for protecting thesemiconductor layered structure 10, so that reliability can be furtherimproved.

A portion of the protective film 13 in the region 12R directly above thelower electrode 12 preferably has a thickness of 40% or less, morepreferably 35% or less, of the thickness of a portion of the protectivefilm 13 on the surface of each of the upper electrodes 11 and on theupper surface of the semiconductor layered structure 10 in the vicinityregions 13R. For example, it is preferable that the thickness of aportion of the protective film 13 in the region directly above the lowerelectrode 12 is 0.2 μm, while the thickness of a portion of theprotective film 13 continuous on the surface of each of the upperelectrode 11 and the upper surface of the semiconductor layeredstructure 10 in the vicinity regions 13R is 0.7 μm. With thisarrangement, deterioration of the semiconductor layered structure 10 inthe vicinity regions 13R of the upper electrodes 11 can be reduced,which allows for maintain reliability, and light from the semiconductorlayered structure 10 can be efficiently extracted.

A portion of the protective film 13 in the region 12R directly above thelower electrode 12 preferably has a thickness of 0.2 to 0.3 μm. Withthis arrangement, absorption of light from the semiconductor layeredstructure 10 can be further reduced while the protective film 13 has theprotection function to some extent. A portion of the protective film 13continuous on the surface of each of the upper electrode 11 and theupper surface of the semiconductor layered structure 10 in the vicinityregions 13R preferably has a thickness of 0.5 to 1.0 μm. With thisarrangement, moisture does not easily pass through the protective film13 and reach the semiconductor layered structure 10. Therefore,deterioration of the semiconductor layered structure 10 can be reduced.Further, the semiconductor layered structure 10 can be protected fromexternal shocks and the like, so that the reliability of thelight-emitting device 100 can be improved.

The protective film 13 is disposed to be continuous on a surface of theextended portion 11 b and on the upper surface of the semiconductorlayered structure 10 including a region in the vicinity of a region onwhich the extended portion 11 b is disposed. Here, a portion of theprotective film 13 on a surface of the extended portion 11 b and aportion of the upper surface of the semiconductor layered structure 10in the region in the vicinity of the region on which the extendedportion 11 b is disposed has a thickness greater than the thickness of aportion of the protective film 13 in the region 12R directly above thelower electrode 12 as shown in FIG. 5. With this arrangement, theeffects of the protective film 13 can be obtained over a wide area ofthe upper surface of the semiconductor layered structure 10, so that thereliability and the light extraction efficiency can be further improved.

As shown in FIG. 2, each of the upper electrode 11 preferably includes afirst upper electrode 111 disposed on a portion of the upper surface ofthe semiconductor layered structure 10 and a second upper electrode 112disposed on the first upper electrode 111. In the case where the upperelectrodes 11 each has such a multilayer structure, an end portion ofthe protective film 13 is preferably disposed between the first upperelectrode 111 and the second upper electrode 112. With this arrangement,entry of moisture via the gap between the first upper electrode 111 andthe protective film 13 can be reduced, so that deterioration of thesemiconductor layered structure 10 can also be reduced, as compared withthe case where the second upper electrode 112 is not disposed. In eachof the upper electrodes 11, such a multilayer structure may be employedonly in a region to be the external connecting portion 11 a. With thisarrangement, absorption of light by the upper electrodes 11 is reduced,so that reduction in output can be reduced.

At the upper surface of the semiconductor layered structure 10, in a topview, the periphery of each of the vicinity regions 13R in the vicinityof the regions on which the upper electrodes 11 are disposed, ispreferably located spaced from an edge of respective one of the upperelectrodes 11 by a distance in a range of 15 μm to 30 μm. That is, theperiphery of a region of the protective film 13 having a thicknessgreater than a thickness of a portion of the protective film 13 in theregion directly above the lower electrode 12 is preferably positioned ina region spaced from an edge of respective one of the upper electrodes11 by a distance in a range of 15 μm to 30 μm. With the vicinity regions13R each having the periphery located spaced from the edge of respectiveone of the upper electrodes 11 by a distance of 15 μm or more,deterioration of the semiconductor layered structure 10 in the vicinityregions 13R can be suppressed. Further, with the vicinity regions 13Reach having the periphery located spaced from the edge of respective oneof the upper electrodes 11 by a distance of 30 μm or less, the lightextraction efficiency can be improved, and deterioration of thesemiconductor layered structure 10 is reduced.

In a region in the vicinity of the region in which the extended portion11 b is disposed, the current density is lower than in a region in thevicinity of the region in which the external connecting portion 11 a isdisposed, and therefore, deterioration of the semiconductor layeredstructure 10 does not easily occur. For this reason, as shown in FIG. 6,in a top view, a portion of the protective film 13 having a largerthickness preferably has a length (for example, Da shown in FIG. 6)extending outside of the extended portion 11 b that is shorter than alength (for example, Db shown in FIG. 6) thereof extending outside ofthe external connecting portion 11 a. For example, a portion of theprotective film 13 having a larger thickness may have a length of 15 μmextending outside of the extended portion 11 b, and may have a length of20 μm extending outside of the external connecting portion 11 a.Reliability of the light-emitting device 100 can be maintained even ifthe area of the protective film 13 having a larger thickness is reducedin the region in the vicinity of the region in which the extendedportion 11 b is disposed. Accordingly, with this arrangement, absorptionof light from the semiconductor layered structure 10 by the protectivefilm 13 can be further reduced, and thus the light extraction efficiencycan be improved.

From the viewpoint of insulation, for example, SiO₂, SiON and SiN can beused for the protective film 13. In the present embodiment, SiO₂ is usedfor the protective film 13.

The protective film 13 can be formed by sputtering, vapor deposition, orthe like. The protective film 13 of the present embodiment which has avaried thickness is formed as below. First, a protective film with apredetermined thickness is disposed on the upper surface of thesemiconductor layered structure 10. Then, a mask is disposed on theprotective film. The mask has an opening at a portion corresponding to aportion of the protective film to have a smaller thickness.Subsequently, the protective film is etched via the mask, and the maskis removed, so that the protective film 13 with a varied thickness canbe formed. Alternatively, first, a protective film of a predeterminedthickness is disposed on the upper surface of the semiconductor layeredstructure 10. Then, a mask is formed on the protective film. The maskhas an opening at a portion corresponding to a portion of the protectivefilm to have a greater thickness. Subsequently, another protective filmis disposed via the mask. Thereafter, the mask is removed, so theprotective film 13 with a varied thickness can be formed. With suchformation methods, a protective film 13 which has a desired thicknesscan be formed.

Insulating members 14 are disposed on the lower surface of thesemiconductor layered structure 10 in regions which include regionsdirectly under the upper electrodes 11 as shown in FIG. 2 and FIG. 3.With this arrangement, current is not easily supplied in the regionsdirectly under the upper electrodes 11, so that, in the semiconductorlayered structure 10, the region other than the regions directly underthe upper electrode 11 serves as a main emission region. Light fromregions in the semiconductor layered structure 10 directly under theupper electrodes 11 is easily reflected or absorbed by the upperelectrode 11, and thus is difficult to extract from the light-emittingdevice 100. Therefore, with the semiconductor layered structure 10 inwhich the region other than the regions directly under the upperelectrodes 11 serves as a main emission region, the light extractionefficiency of the light-emitting device 100 can be improved. In thepresent embodiment, the region in the semiconductor layered structure 10other than the regions therein directly under the upper electrodes 11,that is, a region in the semiconductor layered structure 10 below whichthe lower electrode 12 is disposed, serves as a main emission region,and the thickness of a portion of the protective film 13 in the regiondirectly above the lower electrode 12 is reduced. With this arrangement,the light extraction efficiency of the light-emitting device 100 can beimproved.

As shown in FIG. 3, in a top view, the insulating members 14 aredisposed in regions which include regions directly under the externalconnecting portions 11 a of the upper electrodes 11 and regions directlyunder the extended portions 11 b of the upper electrodes 11. Theinsulating member 14 is disposed in a region directly under the upperelectrode 11, and, in a top view, has an area larger than the total areaof the regions in which the upper electrode 11 is disposed. With thisarrangement, the region in which the above-described effects of theinsulating member 14 are obtained can be increased, so that unevennessin brightness at the light extraction surface of the light-emittingdevice 100 can be reduced.

In the insulating member 14, the same material as used for theprotective film 13 as described above can be used. In the presentembodiment, SiO₂ is used for the protective film 13.

The insulating member 14 preferably has a thickness of 0.05 μm orgreater, more preferably 0.1 μm or greater, for securing insulation.Meanwhile, to reduce the manufacturing cost, the insulating member 14preferably has a thickness of 1 μm or less, more preferably 0.5 μm orless.

A joining member 15 is disposed between the semiconductor layeredstructure 10 and the substrate 16, and serves to join these components.In this arrangement, the lower electrode 12, which is disposed on thesemiconductor layered structure 10, and the substrate 16 areelectrically connected with each other via the joining member 15. Thejoining member 15 can be formed by disposing electrically-conductivemembers over substantially the entirety of the lower surface of thesemiconductor layered structure 10 and the upper surface of thesubstrate 16 and joining these electrically-conductive members together.For the joining member 15, a solder material whose main component is,for example, AuSn, NiSn, AgSn, or the like, is preferably used from theviewpoint of bonding performance and electrical conduction.

The substrate 16 is electrically conductive and is disposed below thesemiconductor layered structure 10. For the substrate 16, for example,CuW, Si, or Mo can be used.

Second Embodiment

A light-emitting device 200 according to the present embodiment isdescribed with reference to FIG. 4.

The light-emitting device 200 is different from the first embodiment inthe structure of the protective film 13. The other configurations arethe same as those of the first embodiment.

As shown in FIG. 4, the protective film 13 is not disposed in the regiondirectly above the lower electrode 12. That is, a portion of the uppersurface of the semiconductor layered structure 10 in the region directlyabove the lower electrode 12 is exposed from the protective film 13.With such a configuration, the light extraction efficiency of thelight-emitting device can be improved as in the first embodiment.Further, light emission from the semiconductor layered structure 10 inthe region on which the protective film 13 is not disposed is notabsorbed by the protective film 13, so that the light-emitting device200 has lower reliability but higher light extraction efficiency thanthat in the first embodiment.

In the description above, first and second embodiments have beendescribed, but the scope of the present invention is not limited tothese embodiments.

While the present invention has been described with respect to exemplaryembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many embodiments other than those specifically described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A light-emitting device, comprising: a semiconductor layered structure; an upper electrode disposed on a portion of an upper surface of the semiconductor layered structure; a lower electrode disposed on a lower surface of the semiconductor layered structure in a region spaced from a region directly under the upper electrode, the lower electrode being reflective; and a protective film disposed continuously on a surface of the upper electrode and on the upper surface of the semiconductor layered structure, wherein the protective film is not disposed in a region directly above the lower electrode.
 2. The light-emitting device according to claim 1, wherein an insulating member is disposed on the lower surface of the semiconductor layered structure in a region that includes the region directly under the upper electrode.
 3. The light-emitting device according to claim 1, wherein the upper electrode includes: an external connecting portion configured to be connected with an external device, and an extended portion extending from the external connecting portion, the protective film is disposed to be continuous on a surface of the extended portion and on the upper surface of the semiconductor layered structure adjacent to a portion on which the extended portion is disposed.
 4. The light-emitting device according to claim 2, wherein the upper electrode includes: an external connecting portion configured to be connected with an external device, and an extended portion extending from the external connecting portion, the protective film is disposed to be continuous on a surface of the extended portion and on the upper surface of the semiconductor layered structure adjacent to a portion on which the extended portion is disposed.
 5. The light-emitting device according to claim 1, wherein the upper electrode includes: a first upper electrode disposed on the portion of the upper surface of the semiconductor layered structure, and a second upper electrode disposed on the first upper electrode, and an end portion of the protective film is located between the first upper electrode and the second upper electrode.
 6. The light-emitting device according to claim 1, wherein the lower electrode is made of Ag or an alloy whose main component is Ag.
 7. The light-emitting device according to claim 1, wherein, in a top view, a periphery of a region of the semiconductor layered structure on which the protective film is disposed is spaced from a periphery of the upper electrode by a distance in a range of 15 μm to 30 μm. 